1. Field of the Invention
The present invention relates to a method for producing a sputtering target assembly consisting of a target member used for sputtering and a backing plate which is bonded to the target member.
2. Description of the Prior Art
The sputtering target is a sputtering source for forming on a substrate an electrode, wiring, element, protection film or the like of various semiconductor devices. In the field of the semiconductor device, materials mainly used as sputtering targets are aluminium, aluminium alloy, metal having a high melting point (such as Ti, TiW, W, Mo or the like), and silicide having a high melting point (such,as MoSi.sub.x, CoSi.sub.x, WSi.sub.x or the like). These materials are normally formed into a disk shape for use as a sputtering target.
When forming a film by sputtering, accelerated particles are applied to a surface of the sputtering target. When the particles collide onto the surface, according to the exchange of the momentum, atoms constituting the sputtering target are discharged into a space. The particles discharged from the sputtering target are accumulated on a substrate placed opposite to the sputtering target, so as to form a thin film.
The sputtering target is normally used in an assembly state with a backing plate bonded to the sputtering target for support and cooling. That is, the sputtering target consists of a target member as a sputtering source in sputtering and a backing plate bonded to the sputtering member. As the backing plate, a metal or alloy having a superior thermal conductivity is used such as OFC (oxygen-free copper), Cu alloy, Al, Al alloy, SUS (stainless steel), Ti or Ti alloy or the like.
When carrying out a sputtering, the sputtering target assembly is mounted on a sputtering apparatus, and the backing plate is cooled. Thus, a heat generated in the target member during the sputtering is absorbed by the backing plate so as to prevent temperature increase of the target member.
Conventionally, bonding between the target member and the backing plate has been carried out mainly by using a soldering material such as In or Sn alloy. However, soldering using soldering materials of low melting point have following defects.
Firstly, the soldering material has a low melting point such as 158.degree. C. of In and 160 to 300.degree. C. of Sn alloys. Consequently, as the temperature during sputtering approaches the melting point, the bonding shear strength is abruptly decreased.
Secondly, the bonding shear strength at the room temperature is as low as 100 kg/cm.sup.2 for In, and 200 to 400 kg/cm.sup.2 for the Sn alloys. Moreover, these are low melting point soldering materials. Consequently, as the temperature increases, the bonding shear strength is further lowered.
Thirdly, when the soldering material is solidified and shrunk during bonding, there is a tendency that pores are left between the target member and the backing plate, which makes it difficult to obtain a 100% bonding without leaving any unbonded portion.
Thus, by the soldering method, it is difficult to bond the target member to the backing plate with a sufficient strength, as this limits to a low level the power applied during sputtering. That is, when using the soldering method, if a sputtering power above a nominal value is applied or if control of a cooling water is insufficient for example, the bonding strength between the target member and the backing plate is lowered as the temperature of the target member increases. Furthermore, if the temperature reaches the melting point of the soldering material, the soldering material is melted and the target member is peeled out from the backing plate. In order to solve these problems, there has been consideration to use, instead of the soldering materials having a low melting point, a brazing material of high melting point. However, if a brazing material of a high melting point is used, the brazing process requires a high temperature, which adversely affects the quality of the target member. That is, a brazing material of a high melting point cannot be used in practice.
As has been described above, the soldering method limits to a low level the power applied for sputtering. On the other hand, in order to improve a throughput of a film forming by sputtering, there is a tendency to increase the power for sputtering. For this, it is desired to prepare a sputtering target assembly which can be maintained at a predetermined bonding strength between the target member and the backing plate even under a high temperature.
In order to achieve this, Japanese Patent Laid-Open Hei 4-143269 discloses a method for bonding a target member with a backing plate directly or via a spacer having a higher melting point than the target member. Here, the bonding between the target member and the backing plate into a unitary body is carried out by way of an explosive bonding and other methods such as hot press method, HIP method, and hot roll method.
However, these methods require that the target member be bonded with the backing plate under a significantly large shock or load. The target member is deformed, which in turn causes an internal distortion and tissue change as well as contamination of the surface portion. Consequently, these methods cannot be applied to a target member in which a crystal grain size and crystalline axis direction are adjusted.
Moreover, Japanese Patent Laid-Open Hei 6-158296 discloses a method of bonding a target member made from Ti having a crystal grain size of 100 .mu.m or below with a backing plate made from Ti by way of solid phase diffusion bonding. More specifically, the target member is bonded to the backing plate in a vacuum under the conditions of: strain rate 1.times.10.sup.-3 /sec or below; pressure applied 0.1 to 20 kg/mm.sup.2 ; and the temperature 350 to 650.degree.C. Japanese Patent Laid-Open Hei 6-158296 mentions that this method enables to obtain a 100% bonding of a high adherence strength and a high bonding strength without leaving any unbonded portion.
Actually, however, the solid phase diffusion boding is still insufficient to obtain a sufficient bonding strength. Especially, it is difficult to obtain a sufficient tensile strength.